# std::ranges::partition

From cppreference.com
< cpp‎ | algorithm‎ | ranges

Defined in header `<algorithm>` |  |
Call signature |  |
template< std::permutable I, std::sentinel_for<I> S, class Proj = std::identity, std::indirect_unary_predicate<std::projected<I, Proj>> Pred > constexpr ranges::subrange<I> partition( I first, S last, Pred pred, Proj proj = {} ); | (1) | (since C++20)
template< ranges::forward_range R, class Proj = std::identity, std::indirect_unary_predicate< std::projected<ranges::iterator_t<R>, Proj>> Pred > requires std::permutable<ranges::iterator_t<R>> constexpr ranges::borrowed_subrange_t<R> partition( R&& r, Pred pred, Proj proj = {} ); | (2) | (since C++20)

1) Reorders the elements in the range `[`first`,` last`)` in such a way that the projection proj of all elements for which the predicate pred returns true precede the projection proj of elements for which predicate pred returns false. Relative order of elements is not preserved.
2) Same as (1), but uses r as the source range, as if using ranges::begin(r) as first and ranges::end(r) as last.

The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:

- Explicit template argument lists cannot be specified when calling any of them.
- None of them are visible to argument-dependent lookup.
- When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

### Parameters

first, last | - | the range of elements to reorder
r | - | the range of elements to reorder
pred | - | predicate to apply to the projected elements
proj | - | projection to apply to the elements

### Return value

A subrange starting with an iterator to the first element of the second group and finishing with an iterator equal to last. (2) returns std::ranges::dangling if r is an rvalue of non-`borrowed_range` type.

### Complexity

Given N = ranges::distance(first, last), exactly N applications of the predicate and projection. At most N / 2 swaps if `I` models ranges::bidirectional_iterator, and at most N swaps otherwise.

### Possible implementation

    struct partition_fn
    {
        template<std::permutable I, std::sentinel_for<I> S, class Proj = std::identity,
                 std::indirect_unary_predicate<std::projected<I, Proj>> Pred>
        constexpr ranges::subrange<I>
            operator()(I first, S last, Pred pred, Proj proj = {}) const
        {
            first = ranges::find_if_not(first, last, std::ref(pred), std::ref(proj));
            if (first == last)
                return {first, first};

            for (auto i = ranges::next(first); i != last; ++i)
            {
                if (std::invoke(pred, std::invoke(proj, *i)))
                {
                    ranges::iter_swap(i, first);
                    ++first;
                }
            }
            return {std::move(first), std::move(last)};
        }

        template<ranges::forward_range R, class Proj = std::identity,
                 std::indirect_unary_predicate<
                     std::projected<ranges::iterator_t<R>, Proj>> Pred>
        requires std::permutable<ranges::iterator_t<R>>
        constexpr ranges::borrowed_subrange_t<R>
            operator()(R&& r, Pred pred, Proj proj = {}) const
        {
            return (*this)(ranges::begin(r), ranges::end(r),
                           std::ref(pred), std::ref(proj));
        }
    };

    inline constexpr partition_fn partition;

### Example

Run this code

    #include <algorithm>
    #include <forward_list>
    #include <functional>
    #include <iostream>
    #include <iterator>
    #include <ranges>
    #include <vector>

    namespace ranges = std::ranges;

    template<class I, std::sentinel_for<I> S, class Cmp = ranges::less>
    requires std::sortable<I, Cmp>
    void quicksort(I first, S last, Cmp cmp = Cmp {})
    {
        using reference = std::iter_reference_t<I>;

        if (first == last)
            return;

        auto size = ranges::distance(first, last);
        auto pivot = ranges::next(first, size - 1);
        ranges::iter_swap(pivot, ranges::next(first, size / 2));

        auto tail = ranges::partition(first, pivot, [=](reference em)
        {
            return std::invoke(cmp, em, *pivot); // em < pivot
        });

        ranges::iter_swap(pivot, tail.begin());
        quicksort(first, tail.begin(), std::ref(cmp));
        quicksort(ranges::next(tail.begin()), last, std::ref(cmp));
    }

    int main()
    {
        std::ostream_iterator<int> cout {std::cout, " "};

        std::vector<int> v {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
        std::cout << "Original vector:  \t";
        ranges::copy(v, cout);

        auto tail = ranges::partition(v, [](int i) { return i % 2 == 0; });

        std::cout << "\nPartitioned vector: \t";
        ranges::copy(ranges::begin(v), ranges::begin(tail), cout);
        std::cout << "│ ";
        ranges::copy(tail, cout);

        std::forward_list<int> fl {1, 30, -4, 3, 5, -4, 1, 6, -8, 2, -5, 64, 1, 92};
        std::cout << "\nUnsorted list: \t\t";
        ranges::copy(fl, cout);

        quicksort(ranges::begin(fl), ranges::end(fl), ranges::greater {});
        std::cout << "\nQuick-sorted list: \t";
        ranges::copy(fl, cout);

        std::cout << '\n';
    }

Possible output:

    Original vector:        0 1 2 3 4 5 6 7 8 9
    Partitioned vector:     0 8 2 6 4 │ 5 3 7 1 9
    Unsorted list:          1 30 -4 3 5 -4 1 6 -8 2 -5 64 1 92
    Quick-sorted list:      92 64 30 6 5 3 2 1 1 1 -4 -4 -5 -8

### See also

ranges::partition_copy (C++20) | copies a range dividing the elements into two groups (algorithm function object)
ranges::is_partitioned (C++20) | determines if the range is partitioned by the given predicate (algorithm function object)
ranges::stable_partition (C++20) | divides elements into two groups while preserving their relative order (algorithm function object)
partition | divides a range of elements into two groups (function template)

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